Crystalline Texture

Image credit: Scott Brande

Classification of the Crystalline Igneous Rocks

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Crystalline Textures

Crystal sizes in igneous rocks

Molten rock may cool more slowly when confined to underground environments, and more quickly when erupted at the surface of the earth. The rate at which molten rock cools controls the eventual sizes of crystals in igneous rock.

  • Molten rock that cools at a near-uniform rate typically forms a rock with a single population of crystal sizes.
  • Molten rock that cools in different environments may experience periods of faster and slower cooling that results in multiple populations of crystal sizes.

Crystal Size

Phaneritic and Aphanitic Textures

The rate at which magma cools determines in part the final sizes of crystals in the solid rock. The terms phaneritic and aphanitic refer to the qualitative judgement we make when viewing an igneous rock with the eye unaided by a magnifying lens.

Aphanitic texture -invisible sized crystals

  • Faster cooling rates occur at the surface of the earth when molten rock erupts, or underground when molten rock encounters much cooler rock.
  • Faster crystallization rates result in microscopic crystal sizes.
  • Microscopic crystal sizes are termed aphanitic.

Phaneritic texture - visible sized crystals

  • Slower cooling rates occur at below the surface of the earth where molten rock pools without eruption.
  • Slower crystallization rates result in larger crystal sizes, typically visible to the naked eye (macroscopic size).
  • Macroscopic crystal sizes are termed phaneritic.

Phaneritic Granite and Aphanitic Rhyolite

  • Granite and rhyolite are both felsic in composition - both are lighter-toned.
  • Both are crystalline.
  • Granite and rhyolite are distinguished by crystal size.
    • granite is phaneritic (crystals large enough to be seen without magnification)
    • rhyolite is aphanitic (microscopic crystals not visible to the naked eye)

Watch this short video to learn how to observe and recognize these differences needed for identification.

Phaneritic Gabbro and Aphanitic Basalt

  • Gabbro and basalt are both mafic in composition - both are darker-toned.
  • Both are crystalline.
  • Gabbro and basalt are distinguished by crystal size.
    • gabbro is phaneritic (crystals large enough to be seen without magnification)
    • basalt is aphanitic (microscopic crystals not visible to the naked eye)

Watch this short video to learn how to observe and recognize these differences needed for identification.

Multiple Crystal Size Populations

Porphyritic Texture

  • Porphyritic texture is recognized by two distinct populations of crystal sizes.
  • When an igneous rock exhibits phaneritic crystals (visible to the naked eye) floating in a finer-grained matrix (either smaller phaneritic or aphanitic texture), the rock texture is termed porphyritic.

Watch this short video to learn how to observe and recognize porphyritic texture.